New: Allow to create MImServer in IC and embedd plugin widget in application

RevBy: Michael Hasselmann, Jan Arne Petersen
If running using QT_IM_MODULE=MaliitDirect the input-context
will instantiate the Maliit server (MImServer)
The application can then use Maliit::InputMethod::pluginWidget() to
get the plugin widget, and insert this into its widget hierachy. This
allows for Maliit to be hosted entirely within the application process.

Note: Currently the available reference plugins misbehave slightly when embedded
in this way as they assume to be embedded into a full-screen window.